Senior Airman Michael Kearns had been back from Iraq for only two months when he was pulled over on a Florida highway for going more than 120 miles per hour on his new Suzuki. He knew his motorcycle riding was reckless, but after living through daily mortar attacks on his base in Iraq, he said he needed the adrenaline rush. “When you get here, there’s nothing that’s very exciting that keeps your pulse going,” Airman Kearns, 27, said in a recent interview. His experience is so common that the United States military, alarmed by a rising suicide rate and the...